For example, a gas which is discharged from a combustion engine such as an internal-combustion engine and an incinerator becomes a mixture gas obtained by mixing various gaseous materials with one another. Such a flue gas flows inside a pipe line, and is supplied (discharged) to a predetermined device or an atmosphere. Here, as a method of measuring a concentration of an ammonia compound contained in the flue gas, there is known a method of wet-sampling the flue gas, collecting an ammonia element in a solution, and analyzing the collected solution by an indophenol absorption spectrophotometry, an ion chromatography analysis, or the like. As an example, “JISK0099” discloses a “method of analyzing ammonia in a flue gas”.
Further, as a method of measuring a concentration of a specific material contained in the mixture gas (which is mainly a circulating gas) flowing inside the pipe line, there is known a method of causing a laser beam to pass through a predetermined path of a pipe line and measuring a concentration of a specific material in a measurement subject from the input and output thereof. For example, the present applicant has proposed Patent Literature 1, which discloses a gas concentration measurement device including a light source which oscillates a laser beam of an original absorption wavelength to a gaseous material as a measurement subject, a modulation unit which modulates the oscillation wavelength of the laser beam oscillated from the light source into at least two different frequencies, a guide unit which guides the laser beam modulated by the modulation unit to a measurement region where the gaseous material exists, a light receiving unit which receives the laser beam transmitted, reflected, or scattered in the measurement region, and a plurality of phase sensitive detectors which sequentially demodulate each of the modulated signals for each frequency in the signal received by the light receiving unit.